Because related power metal oxide semiconductor field effect transistors (MOSFET) have an input impedance higher than a bipolar transistor, it has a large power gain and a simple gate driving circuit. Also because a power MOSFET is a unipolar device, it has no time delay due to the accumulation or recombination of minority carriers while it is turned-off.
Therefore, the application of the power MOSFET in such circuits as a switching mode power supply apparatus, a lamp stabilization and motor driving circuit, etc, has gradually expanded. As for the structure of a related power MOSFET, a double diffused MOSFET structure using a planar diffusion technology has been used. One related representative device using the DMOSFET structure is an LDMOS device.
A photo resist is used as a blocking mask at the time of forming a P type body of the LDMOS device. The photo resist has an effect on a channel region at the time of an ion implantation process into a buried layer that is processed with high energy, such that it is difficult to uniformly control Vt of the device.